It is believed that methane is produced during the conversion of peat to coal. The conversion is believed to be a result of naturally occurring thermal and biogenic processes. Because of the mutual attraction between the carbonaceous matrix of coal and the methane molecules, a large amount of methane can remain trapped in-situ as gas adhered to the carbonaceous products formed by the thermal and biogenic processes. In addition to methane, lesser amounts of other compounds such as water, nitrogen, carbon dioxide, and heavier hydrocarbons, and sometimes small amounts of other fluids such as argon and oxygen, can be found within the carbonaceous matrix of the formation. The gaseous fluids which are produced from coal formations collectively are often referred to as "coalbed methane." Coalbed methane typically comprises more than about 90 to 95 volume percent methane. The reserves of such coalbed methane in the United States and around the world are huge. Most of these reserves are found in coal beds, but significant reserves may be found in gas shales and other solid carbonaceous subterranean formations which are also believed to have resulted from the action of thermal and biogenic processes on decaying organic matter.
Methane is the primary component of natural gas, a widely used fuel source. Coalbed methane is now produced from coal seams for use as a fuel. Typically, a wellbore is drilled which penetrates one or more coal seams. The wellbore is utilized to recover coalbed methane from the seam or seams. The pressure difference between a coal seam and the wellbore provides the driving force for flowing coalbed methane to and out of the wellbore. Reduction of pressure in the coal seam as coalbed methane is produced increases desorption of methane from the carbonaceous matrix of the formation, but, at the same time, deprives the system of the driving force necessary to flow coalbed methane to the wellbore. Consequently, this method loses its effectiveness over time for producing recoverable coalbed methane reserves. It is generally believed that this method is only capable of economically producing about 35 to 70% of the methane contained in a coal seam.
An improved method for producing coalbed methane is disclosed in U.S. Pat. No. 5,014,785 to Purl, et al. In this process, a methane-desorbing gas such as an inert gas is injected into a solid carbonaceous subterranean formation through at least one injection well, with a methane-containing gas recovered from at least one production well. The desorbing gas, preferably nitrogen, mitigates depletion of pressure within the formation and is believed to desorb methane from the carbonaceous matrix of the formation by decreasing the methane partial pressure within the formation. This method is effective for increasing both the total amount and rate of methane production from a solid carbonaceous subterranean formation such as a coal seam. Present indications are that the rate of methane production can be increased and that the total amount of methane recovered can be increased substantially, to possibly 80% or more of the methane contained in the formation.
As will be demonstrated by an Example contained herein, long-term injection of an inert gas into a formation may result in the production of a methane-containing gas having an inert gas fraction that generally increases in volume percent with time. This result may be undesirable as it may be necessary to lessen the concentration of injected inert gas in the produced methane-containing mixture before the mixture can be transferred into a natural gas pipeline or otherwise utilized.
What is needed is an improved process for the recovery of methane from solid carbonaceous subterranean formations that can provide a methane-containing gas that contains as little of the injected inert gas as possible to mitigate the costs associated with removing the injected gas from the produced methane-containing gaseous mixture.
As used herein, the following terms shall have the following meanings:
(a) "Air" refers to any gaseous mixture containing at least 15 volume percent oxygen and at least 60 volume percent nitrogen. "Air" is preferably the atmospheric mixture of gases found at the well site and contains between about 20 and 22 volume percent oxygen and between about 78 and 80 volume percent nitrogen. PA1 (b) "Cleats" or "cleat system" is the natural system of fractures within a solid carbonaceous subterranean formation. PA1 (c) "Adsorbate" is that portion of a gaseous mixture which is preferentially adsorbed by a bed of adsorptive material during the adsorptive portion of a pressure swing adsorption separator's cycle. PA1 (d) "Formation paring pressure" and "parting pressure" mean the pressure needed to open a formation and propagate an induced fracture through the formation. PA1 (e) "Fracture half-length" is the distance, measured along the fracture, from the wellbore to the fracture tip. PA1 (f) "Recovering" means a controlled collection and/or disposition of a gas, such as storing the gas in a tank or distributing the gas through a pipeline. "Recovering" specifically excludes venting the gas into the atmosphere. PA1 (g) "Reservoir pressure" means the pressure of a productive formation near a well during shut-in of that well. The reservoir pressure of the formation may change over time as inert methane-desorbing gas is injected into the formation. PA1 (h) "Solid carbonaceous subterranean formation" refers to any substantially solid, methane-containing material located below the surface of the earth. It is believed that these methane-containing materials are produced by the thermal and biogenic degradation of organic matter. Solid carbonaceous subterranean formations include but are not limited to coalbeds and other carbonaceous formations such as shales. PA1 (i) "Well spacing" or "spacing" is the straight-line distance between the individual wellbores of a production well and an injection well. The distance is measured from where the wellbores intercept the formation of interest. PA1 (j) "Preferentially adsorbing," "preferentially adsorbs," and "preferential adsorption" refer to processes that alter the relative proportions of the components of a gaseous fluid. The processes fractionate a mixture of gases by equilibrium separation, kinetic separation, steric separation, and any other process or combinations of processes which within a bed of material would selectively fractionate a mixture of gases into an oxygen-depleted fraction and an oxygen-enriched fraction. PA1 (k) "Raffinate" refers to that portion of the gas injected into a bed of adsorptive material which is not preferentially adsorbed by the bed of adsorptive material. PA1 (l) "Standard initial production rate" as used herein refers to the actual or predicted methane-containing gas production rate of a production well immediately prior to flowing a methane-desorbing gas through the well to increase its production rate. A standard initial production rate may be established, for example, by allowing a well to operate as a pressure depletion well for a relatively short period of time just prior to inert gas injection. The standard initial production rate can then be calculated by averaging the production rate over the period of pressure depletion operation. If this method is used, the well preferably will have been operated long enough that the transient variations in production rates do not exceed about 25% the average production rate. Preferably, the "standard initial production rate" is determined by maintaining constant operating conditions, such as operating at a constant bottom hole flowing pressure with little or no fluid level. Alternatively, a "standard initial production rate" may be calculated based on reservoir parameters, as discussed in detail herein, or as otherwise would be calculated by one of ordinary skill in the art. PA1 (m) "Inert methane-desorbing gas" as used herein refers to any gas or gaseous mixture that contains greater than fifty volume percent of a relatively inert gas or gases. A relatively inert gas is a gas that promotes the desorption of methane from a solid carbonaceous subterranean formation without being strongly adsorbed to the solid organic material present in the formation or otherwise chemically reacting with the solid organic material to any significant extent. Examples of relatively inert gases include nitrogen, argon, air, helium and the like, as well as mixtures of these gases. An example of a strongly desorbed gas not considered to be a relatively inert gas is carbon dioxide. PA1 (n) "Reacted" as used herein refers to any reaction of an oxygen-enriched stream with a second process stream. Examples of such reactions include but are not limited to combustion, as well as other chemical reactions including reforming processes such as the steam reforming of methane to synthesis gas, oxidative chemical processes such as the conversion of ethylene to ethylene oxide, and oxidative coupling processes as described herein. PA1 (o) "Oxidizable reactant" as used herein means any organic or inorganic reactant that can undergo chemical reaction with oxygen. For example, oxidizable reactants include materials which can be chemically combined with oxygen, that can be dehydrogenated by the action of oxygen, or that otherwise contain an element whose valence state is increased in a positive direction by interaction with oxygen. PA1 (p) "Organic reactant" as used herein means any carbon and hydrogen-containing compound regardless of the presence of heteroatoms such as nitrogen, oxygen and sulfur. Examples include but are not limited to methane and other hydrocarbons whether used as combustion fuels or starting materials for conversion to other organic products. PA1 (q) "Inorganic reactant" as used herein means any reactant which does not contain both carbon and hydrogen. PA1 (r) "Methane-desorbing gas volume percent" refers to the volume percent of the inert methane-desorbing gas found in the produced methane-containing gaseous mixture at a given point in time that is attributable to the injection of the methane-desorbing gas. It should be noted that if a multi-component inert methane-desorbing gas is used, some components of the gas may appear in the produced gas before others or in varying ratios. In this case, the methane-desorbing gas volume percent refers to the sum of all inert gas components actually appearing in the produced gas. If the formation produces any naturally-occurring inert gas components identical to one or more components injected into the formation, the naturally-occurring portion of the components should be subtracted from the detected amount to determine the methane-desorbing gas volume percent attributable to inert gas injection. PA1 (s) "Formation location" refers to a location within a solid carbonaceous subterranean formation into which an inert methane-desorbing gas can be injected to increase methane-containing gas production from a production well in fluid communication with the point of gas injection. Inert gas typically is injected from the surface into such a location through one or more injection wells bored into the formation. PA1 (t) "Enhanced production rate" for a given well is any rate greater than the standard initial production rate which is caused by the injection of an inert methane-desorbing gas into the formation. In most cases, it is believed that the enhanced production rate of the well will remain greater than the standard initial production rate of the well for a substantial period of time following the suspension of inert methane-desorbing gas injection or a reduction of inert gas injection rate, thereby retaining some of the advantages of enhanced production at a reduced methane-desorbing gas volume percent. Where the term "fully-enhanced production rate" is used, the term refers to the maximum steady-state production rate caused by continuously injecting the inert methane-desorbing gas into the formation at a given injection rate. PA1 (u) "Methane-derived reactant" means a compound created directly from a methane-containing feedstock, a compound whose synthesis employs an intermediate compound created from a methane-containing process stream, or a non-inert contaminating compound co-produced with natural gas. Examples of methane-derived reactants include but are not limited to synthesis gas obtained by reforming methane, methanol or dimethyl ether when formed by the direct or step-wise reaction of synthesis gas over a catalyst, mixtures containing C.sub.2 and greater hydrocarbons and/or heteroatom-containing variants thereof obtained from a process such as a Fischer-Tropsch catalytic hydrogenation of methane-derived synthesis gas over a catalyst, and the common natural gas contaminant hydrogen sulfide.